1. Field of the Invention
The field of the invention is baghouse filtering systems for filtering particulates from the flue exhaust of large scale producers of smokestack emissions.
2. Description of the Related Art
In the realm of methods and means for removing particulate matter from the flue exhaust of large scale producers of emissions, such as utility power plants and the like, extensive research and money has gone into providing various types of apparatus and equipment to accomplish this task. In this respect, construction of so called "baghouses" situated in the flue line intermediate the power plant boiler or furnace and the smokestack has evolved wherein are contained rather large numbers of long (thirty to forty-five feet or so), cylindrical (eight to twelve inches in diameter) bag filters through whose cylindrical walls must pass the gases of the flue exhaust of the coal or oil burning furnace or other process on their trip to the smokestack. These bag filters have spaced apart anti-collapse circumferential rings sewed into their sides to generally maintain a round tube.
The bag filters are usually oriented vertically and in one common application, the flue exhaust is directed to the interior of the cylindrical bags whereupon the gases of the flue exhaust pass through the woven cloth material of the bags leaving the solid particulate matter behind and interiorly to the bags.
The baghouse is divided into a number of sealed compartments, each compartment containing a relatively large number of bag filters, in the range of one hundred to six hundred individual bags. The compartments of the baghouse are arranged so as to be in parallel with each other in the flue exhaust line, i.e., the flue exhaust line from the power plant boiler is manifolded into the inlets of all compartments, and the outlets of the compartments are manifolded back into the exhaust gases line leading to the smokestack. In each compartment, a hopper resides at the bottom to contain the particulate matter which falls out of the flue exhaust and which is collected by the bag filters.
In the development of baghouse construction, several primary methods of filtering have evolved. The most common is the bottom inlet configuration wherein the flue exhaust from the power plant boiler is introduced into the bottom opening of vertically oriented bag filters, the exhaust gases with particulates rising interiorly in the bag filter with the gases of the flue exhaust passing horizontally through the cylindrical sides of the filter where the gas is collected and moved on to the smokestack. To accomplish this, the bottom ends of the bag filters are attached to what is termed the lower or bottom tubesheet situated in the lower region of a compartment, the tubesheet being a rather large flat surface plate which engages the walls of the compartment in a sealed manner, the tubesheet having a number of round perforations therethrough. The bottom of each bag filter engages one of the round perforations also in a sealed manner. The bags are suspended from near the top of the baghouse by means of a round plate whose circumference mates with the top of the bag filter. The round plate has a centrally located eye bolt which attaches to one end of a spring, the other end of the spring fastened to supports below the roof of the baghouse. By such means, the bag filter is suspended under tension. Below the lower tubesheet is the hopper portion of the compartment, the hopper holding the particulate matter for removal, a lower plenum being formed between the bottom tubesheet and the hopper. A filter plenum is similarly formed with the walls of the compartment above the lower tubesheet, the bag filters residing in this plenum.
The flue exhaust is introduced into this lower plenum whereupon the flue exhaust and particulate matter rise vertically in the bag filter with gases passing through the bag filter walls into the filter plenum. The filtered gases are drawn from the filter plenum to the smokestack.
The second method of baghouse filtering is the top inlet method where in configuration, in addition to the bottom tubesheet and hopper, an upper or top tubesheet is added in the upper portion of the compartment, the upper tubesheet being similar in construction to the lower tubesheet, i.e., engaging the walls of the compartment in a sealed manner and having a plurality of round openings therethrough. Just as apparent, an upper or top plenum is formed between the top surface of the upper tubesheet, and the walls and roof of the compartment. Each bag filter is now configured to join an opening of the top tubesheet in a sealed manner as they do the openings of the bottom tubesheet. Consequently, the filter plenum surrounding the sides of the bag filters is now defined by both the lower and upper tubesheet and walls of the compartment, the filter plenum situated between the top plenum and the bottom plenum.
In the top inlet configuration, the flue exhaust from the power plant boiler is introduced into the top plenum above the upper tubesheet in order that the flue exhaust proceed downward through the individual bag filters. The smaller particulate matter is trapped by the woven material cylindrical sides of the bag filter as the gases of the flue exhaust pass horizontally through the filters. The exhaust gases are collected in the filter plenum between the lower and the upper tubesheet and moved on to the smokestack.
In both configurations of baghouse construction, a sufficient number of compartments to receive the flue exhaust from a given power plant are available to place the smokestack emissions in regulatory compliance. In addition, there are one, or in some designs, two more additional compartments. These extra compartments serve to allow for any one of the compartments to be off-line for any reason, (such as maintenance) and for any other compartment to be off-line while it is being cleaned. The extra compartment for cleaning off-line is always included; the maintenance compartment is optional. Cleaning of a compartment is required so that the collected particulate matter on the inside surface of the bag filter may be removed to the hopper below. Different methods for cleaning each of the bag filters is available, such as reversing the gas direction so that clean gases from the filtered flue exhaust obtained from the filter plenum of other on-line compartments enters the filter plenum surrounding the outside walls of the bag filters. In such a case, the "reverse air" gases deform the bag filters about the spaced apart anti-collapse circumferential rings and by doing so dislocate the particulate matter or cake (build-up of particulate matter on the inside of the bag filter) off of the bag material.
The particulates which exist in a baghouse may range in size from rather large particulates of one hundred micrometers in diameter or so (resulting, for example, from burning eastern high sulfur coal) to particulates which are fractions of a micrometer in diameter. The very small sub-micrometer particulates do not fall naturally out of the non-moving flue exhaust in the time span during which the flue exhaust is in the baghouse.
Conventional reverse air cleaning cycles include one or more "null periods" which are short duration periods when there is no flow in either direction through the bags. Null periods are intended to allow time for the dislodged cake to fall to the hopper under that compartment. In addition to any null periods that may be included earlier in the cleaning cycle there will always be a final null period between the moment when reverse flow stops and the moment when forward flow starts. In a typical cleaning cycle the duration of the final null period is usually on the order of 30 to 40 seconds.
During this final null period, just prior to returning to service, the bags and the upper part of the hopper will contain astronomical populations of extremely small particles that will either settle very slowly or will float. The particles that are large enough to settle will move downward so slowly that the fastest settlers will require minutes to reach the hopper and the slowest will require months to reach the hopper. The smallest particles, those smaller than a micrometer, will not settle; they will float and move about in Brownian movement just like dust particles in a beam of light. The too-slow settling particles together with the floating particles are referred to in this description as "floaters".
It is a well established principle in filtration that pressure loss increases as particle size decreases. If the floaters are not removed prior to returning the bags to service after reverse air cleaning they will return to the bags and comprise a layer of extremely small particles in the filter cake. When a group of bags is returned to service after reverse air cleaning the pressure differential between the clean side and the dirty side of the bags is known in the art as the "residual pressure loss". Although the contribution of the layer of floaters to the residual pressure loss has not been determined it can be determined experimentally and can be expected to be a significant component of the residual pressure loss.
In the literature there are reports of residual pressure losses for this type of baghouse on the order of 1.5 to 2.0 inches of water column gauge ("wg). There are also reports of residual loss in the 5 to 7" wg range. Purging the floaters may provide a small (less then 1" wg) reduction for applications that achieve the low end of residual loss and can be expected to provide a large (greater than 1" wg) reduction in residual pressure loss for many applications.
The subject patent however, presents an improvement to top inlet systems of baghouse construction in a means and method by which floaters which have not been trapped may be purged. An alternate embodiment applies the concepts embodied in the invention to the more common bottom inlet baghouse construction.